Vehicle-mounted systems include an engine system having a fuel vapor processing unit. In the engine system, fuel vapor generated in a fuel tank is tentatively collected in a canister without being released to the atmosphere. The collected vapor is purged into an intake passage, and it is combusted in an engine. However, when a fault such as cracking or creation of a hole occurs in, for example, a vapor line or a purge line in the engine system, it is likely that the air tightness of the interior of the processing unit may not be maintained and appropriate processing of fuel vapor is disabled.
Electronic control units are therefore requested to have a capability to diagnose presence or absence of the foregoing fault. However, the internal pressure of a fuel vapor processing system tends to vary during a period in which an engine is in operation or a vehicle is traveling. The foregoing fault cannot be fully diagnosed during that period. Therefore, U.S. Patent No. 2003/0135309 A1 (JP 2003-205798 A) proposes an engine system. In this engine system, after power supply to an electronic control unit is discontinued by the turning off of an ignition switch, the power supply to the electronic control unit is resumed using, a timer in order to perform diagnosis on a fuel vapor processing apparatus.
In the engine system, normal engine control is executed while an engine is in operation and control of diagnosis to be performed on a fuel vapor processing unit is executed after the engine is stopped. The controls are executed mutually independently. Nevertheless, the control structure designed to perform the foregoing diagnosis during execution of a vehicle is employed as it is. An increase in a load of arithmetic operations, which are required for the controls, imposed on the electronic control unit cannot be ignored.
Specifically, in order to effectively utilize a space in an electronic control unit, various programs stored in a single program memory, that is, a sole read only memory (ROM) are selectively executed based on the state of an engine or the state of a vehicle in which the engine is mounted. Thus, the engine control or the control of diagnosis on a fuel vapor processing system is executed. Moreover, programs to be executed in common for the controls are stored in a ROM and used in common. Therefore, the ROM itself is structured to hold, in addition to various programs for executing the two kinds of controls, that is, the normal engine control and the control of diagnosis on a fuel vapor processing system, a mode designation program for determining based on the state of the engine or vehicle whichever of the two controls should be executed for each of the programs.
For execution of the various programs stored in the ROM, the mode designation program is invoked at every time of execution of a program. Consequently, control is executed appropriately according to the state of an engine or a vehicle. However, in practice, the control structure causes a large memory area in the ROM and eventually causes an increase in the load of arithmetic operations on the electronic control unit.